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In CL:1195175 we change the formula for Vaapi VEA. However, the
formula still failed at small resolution. According to the
investigation at crbug.com/889739 comment 1 and 2, using the fixed
buffer size might be the choice before we can update the buffer size
when the bitrate and framerate are changed.

Also, Because this function is general for Vaapi VEA and V4L2 VEA, we
move it to gpu_video_accelerator_util.

BUG=b:113309237
BUG=chromium:889739
TEST=pass VideoEncoderTest CTS on eve and scarlet device
TEST=pass RTC lookback on eve at H264 HD
TEST=pass VEA unittest on eve

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I0aa6fd8b67c19ad93bae6e62430105dc9ccf6a80
Reviewed-on: https://chromium-review.googlesource.com/1253315
Commit-Queue: Chih-Yu Huang <akahuang@chromium.org>
Reviewed-by: Pawel Osciak <posciak@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595738}